1. Field of the Invention
The present invention relates generally to a configuration of a Langevin-type ultrasonic motor, particularly a rod-type ultrasonic motor in which a longitudinal vibration is combined with a torsional vibration, to a system for driving the ultrasonic motor, and to an electronic apparatus utilizing the ultrasonic motor.
2. Description of the Related Art
A conventional rod-type ultrasonic motor has a vibrating body provided with an elastic body for generating a longitudinal vibration and an elastic body for generating a torsional vibration. Drive voltages whose phases are different from each other are applied to the respective elastic bodies to excite the vibrating body, and the force in a torsional direction produced when the vibrating body undergoes an expanding motion caused by combined longitudinal-torsional vibrations is transmitted to a rotor.
In addition to foregoing conventional ultrasonic motor, a standing-wave ultrasonic motor has been proposed in which inclined slit grooves are formed on a vibrating body, torsional vibrations are generated from longitudinal vibrations of a piezoelectric vibrator, and a rotor is rotated by elliptic vibrations that are generated through combination of the longitudinal vibrations and the torsional vibrations.
In other words, when the vibrating body whose surface is in contact with the rotor generates combined longitudinal-torsional vibrations, the force in a torsional direction produced when the vibrating body undergoes an expanding motion is transmitted to the rotor. However, the force in the torsional direction produced when the vibrating body undergoes a contracting motion is not transmitted to the rotor due to reduced frictional force. Hence, the rotor is driven unidirectionally.
Such a conventional ultrasonic motor provided with an elastic body for generating longitudinal vibrations and an elastic body for generating torsional vibrations has a problem in that its configuration and a circuit for supplying electricity are complicated, which results in high cost or a large outer dimension. However, such a conventional ultrasonic motor has a function of controlling the number of revolutions and torque without restraint and thus has an advantage of being adaptable to a wide range of uses.
For instance, a conventional ultrasonic motor shown in FIG. 5 has a standing-wave system. According to this conventional example, there are provided an ultrasonic motor that can rotate bidirectionally with a single phase and a method of driving the ultrasonic motor.
In this ultrasonic motor, elliptic vibrations that are generated through combination of longitudinal vibrations and torsional vibrations are caused at the end face of a vibrating body 120 and thereby a rotor 110 is driven to rotate. The ultrasonic motor includes vibration generation means 122, 124, inclined slit grooves 138, and a voltage application means 150. The vibration generation means 122, 124 generate longitudinal vibrations with a longitudinal resonance frequency or torsional vibrations with a torsional resonance frequency of the vibrating body 120 according to the AC voltage with a frequency to be applied thereto. The inclined slit grooves 138 allow one of a longitudinal vibration and a torsional vibration to be generated from the other and allow them to be combined to generate an elliptic vibration. The voltage application means 150 is different from the two vibration generation means 122, 124 in polarizing direction and selectively switches AC voltages with frequencies corresponding to a longitudinal vibration and a torsional vibration to apply one thus selected.
The vibration generation means 122, 124 generate longitudinal vibrations with a longitudinal resonance frequency or torsional vibrations with a torsional resonance frequency of the vibrating body 120 so that the vibrating body 120 vibrates considerably. In order to generate longitudinal vibrations in the vibrating body 120, a frequency corresponding thereto is selected and an AC voltage with the selected frequency is applied to the vibration generation means 122, 124 by the voltage application means 150. Then torsional vibrations are generated from the longitudinal vibrations generated in the vibrating body 120 through the grooves 138 that are vibration conversion means. Subsequently, the two vibrations are combined to cause elliptic vibrations at the end face of the vibrating body and thereby the rotor rotates. In order to generate torsional vibrations in the vibrating body 120, a frequency corresponding thereto is selected and an AC voltage with the selected frequency is applied to the vibration generation means 122, 124 by the voltage application means. Then longitudinal vibrations are generated from the torsional vibrations generated in the vibrating body 120 through the grooves 138 that are vibration conversion means. Subsequently, the two vibrations are combined to cause elliptic vibrations at the end face of the vibrating body and thereby the rotor rotates. That is, the rotational directions are switched through the use of the fact that the direction of rotation caused when longitudinal vibrations are converted to torsional vibrations is reversed when torsional vibrations are converted to longitudinal vibrations and vice versa.
This switching of rotational directions is operated through switching of frequencies of voltages to be applied to the vibration generation means 122, 124. With respect to these frequencies, a primary resonance frequency of longitudinal vibrations is 55 kHz and a secondary resonance frequency of torsional vibrations is 63 kHz. In other words, this ultrasonic motor has resonance points in modes whose frequencies are different from each other by 8 kHz. Hence, the ultrasonic motor has problems in that due to variations in the two resonance frequencies caused in the production process, variations in characteristics of the ultrasonic motor are caused or two vibration modes influence each other and thereby output power is compelled to be reduced.
Furthermore, there is a problem in that vibrations are unstable, and an ultrasonic motor of a self-oscillation type has difficulty in operational stability. In order to drive this ultrasonic motor, a voltage application unit having a frequency tracking function is required separately. Furthermore, two piezoelectric vibrators are required that are different from each other in polarizing direction. Consequently, there has been a problem in that a size reduction is difficult due to its configuration. If the size reduction is forcibly achieved, power is reduced to an extreme degree.
A Langevin-type ultrasonic motor has a configuration in which a piezoelectric vibrator is sandwiched between rod-like elastic bodies from its both sides. In this Langevin-type ultrasonic motor, while the piezoelectric vibrator is sandwiched between the rod-like elastic bodies from both sides, electrode plates for supplying electricity to the piezoelectric vibrator are required. In addition, it is necessary to convey the torque of the rotor to the outside while maintaining the state where the rotor is in press contact with the elastic body in the end portion of the piezoelectric vibrator. Hence, this kind of ultrasonic motor has to have a complicated support structure and conduction structure and thus it has been difficult to achieve a further size reduction.
This type of Langevin-type ultrasonic motor is shown in FIG. 9 and which includes an ultrasonic motor 16 having a columnar rotor 28 attached with a center shaft 30, a pipe 12, and a holder 14.
The support structure of the conventional ultrasonic motor shown in FIG. 9 is formed with the ultrasonic motor 16 contained inside the pipe 12 and with an end portion of the pipe 12 attached to the holder 14 rotatably. A support member 36 is inserted loosely into a hollow portion formed of a hole made from one end of the center shaft 30 to a portion corresponding to nodes in a torsional vibration mode, and the tip of the support member 36 is fixed to the bottom part of the hollow portion. The other end face of the support member 36 is attached to the inner surface of the holder 14 and the rotor is fixed to the inner surface of the pipe.
In the ultrasonic motor having such a configuration, it is necessary to create the hollow portion formed of a hole made to reach the portion corresponding to nodes and to fix the tip of the support member and the bottom part of the hollow portion to each other. Due to its configuration that requires joining of the rotor and the pipe, the supply of electricity to a piezoelectric vibrator, which is not shown in the drawing, and the like, the manufacturing process is complicated. Hence, it is difficult to achieve a further size reduction. If a size reduction is forcibly achieved, there is a possibility that the structures of portions providing support, pressing forces, and conduction may be stressed, and thereby energy loss may result.